1. Field of the Invention
This invention relates to an optical device, and particularly to an electrically-controllable wave plate and to a polarisation controller including such wave plate.
2. Description of the Related Art
There are two classes of liquid crystal device (LCD) which can be used to control the state of polarisation of light transmitted through the device. Nematic LCDs possess an electrically-controllable birefringence .DELTA.n, and hence a retardation (.delta.), with a fixed optic axis. Such devices have been exploited in a liquid crystal polarisation controller (LCPC) as described in British Patent Publication No. 2202643A. Due to their wavelength of operation-(1550 nm) and the dynamic range required for the retardation, these devices are relatively slow, with response times of several hundred milliseconds. Smectic LCDs, in particular those using chiral smectic A liquid crystal materials, may exhibit the electroclinic effect in which the optic axis of the LC layer rotates in response to the applied voltage. Such a device therefore exhibits a fixed retardation with an electrically-controllable orientation of optic axis. The electroclinic effect is very fast, with response times of only a few microseconds, but has very limited dynamic range and so is not a feasible basis for a useful polarisation controlling device.
The existing LCDs therefore have limitations on their use as polarisation controlling elements, because only either the birefringence or the optic axis orientation is controllable; the other is fixed during the fabrication of the device.
A polarisation controller should be endless, i.e. the device should not reach the limits of its range and require resetting. Clearly the electroclinic device in which the orientation of the optic axis is controlled does not meet this requirement, because of the limited angular range of the effect (typically .+-.15.degree.).
The slow response of the nematic, electrically-controlled birefringence LCPC, mentioned above severely restricts its operation capability, and devices fifty to one hundred times faster are required.